Semi-empirical analysis of SDSS galaxies: I. Spectral synthesis method

In this paper we investigate the power of spectral synthesis as a mean to estimate physical properties of galaxies. Spectral synthesis is nothing more than the decomposition of an observed spectrum in terms of a superposition of a base of simple stellar populations of various ages and metallicities, producing as output the star-formation and chemical histories of a galaxy, its extinction and velocity dispersion. Our implementation of this method uses the Bruzual & Charlot (2003) models and observed spectra in the 3650--8000 \AA range. The reliability of this approach is studied by three different means: (1) simulations, (2) comparison with previous work based on a different technique, and (3) analysis of the consistency of results obtained for a sample of galaxies from the SDSS. We find that spectral synthesis provides reliable physical parameters as long as one does not attempt a very detailed description of the star-formation and chemical histories. We show that besides providing excellent fits to observed galaxy spectra, this method is able to recover useful information on the distributions of stellar ages and, more importantly, stellar metallicities. Stellar masses, velocity dispersion and extinction are also found to be accurately retrieved for realistic signal-to-noise ratios. We apply this synthesis method to a volume limited sample of 50362 galaxies from the SDSS DR2, producing a catalog of stellar population properties. A comparison with recent estimates of both observed and physical properties of these galaxies obtained by other groups shows good qualitative and quantitative agreement. The confidence in the method is further strengthened by several empirical and astrophysically reasonable correlations between synthesis results and independent quantities.